This invention relates to cardiac pacing systems and, more particularly, pacing systems having a capability of detecting and treating long QT syndrome.
It is known that prolongation of the QT interval frequently results in serious ventricular arrhythmias, and might be a predictor of torsades de pointes (TdP) and sudden death. See, xe2x80x9cElectrophysiology of Torsades de Pointes,xe2x80x9d Fontaine et al., World Symposium on Cardiac Pacing, 6th, Montreal, PACESYMP, 1979, p. 6.3. As set forth in the Fontain et al. article, TdP is manifested by an ECG pattern which occurs as a transient life-threatening ventricular arrhythmia, frequently announced by bradycardia, long QT intervals, very large T waves and premature ventricular contractions (PVCs). The term Ventricular Extra Systole (VES) is used herein synonymously with PVC. The premature ventricular beats that occur at the onset of long QT syndrome appear around the end of the large T waves, and the number of such VESs increases with time, leading to couplets or triplets and eventually to a degeneration of the rapid arrhythmia which characterizes TdP. A VES originates in the ventricle, and is considered a PVC when it comes xe2x80x9cearlyxe2x80x9d with respect to the previous VS.
QT prolongation as a predictor of the onset of ventricular arrhythmia is known in the literature. See xe2x80x9cQT-Sensitive Cybernetic Pacemaker: A New Role For An Old Parameter?xe2x80x9d, Puddu and Torresani, PACE, Vol. 9, January-February 1986, Part 1, pp. 108-123. Overdrive pacing is suggested as an effective therapeutic tool in both congenital and acquired QT prolongation. See also U.S. Pat. No. 5,217,393, disclosing a pacemaker wherein during each cardiac cycle the QT wave form is monitored and integrated and a ventricular gradient is obtained. Overdrive pacing is triggered when the gradient increases above a predetermined threshold. Although the literature identifies certain features of the cardiac signal which may be predictors of the onset of the ventricular arrhythmia such as TdP and ventricular fibrillation, often leading to syncope and sudden death, what is needed in the art is a pacemaker which systematically acquires data and processes it so as to be able to determine, with a high degree of statistical probability exactly when there is an onset of ventricular arrhythmia. For example, QT prolongation by itself is likely, for most patients, to be an insufficient predictor of a true onset of TdP or another ventricular arrhythmia. Rather, what is required is processing of sensed cardiac signal data, the processing being done systematically so as to make a continuous determination of the probability of the onset of such a ventricular arrhythmia, thereby enabling effective pacing treatment of the patient condition.
More broadly, there is a need in the art for providing a more systematic and reliable means of determining when patient conditions suggest the onset of a dangerous ventricular arrhythmia, and for providing an effective overdrive pacing therapy to prevent such arrhythmia. For instance, it is known that patients can be vulnerable to ventricular tachycardia (VT) during the awakening hours, while TdP emerging from Long QT syndrome and other ventricular arrhythmias can occur at any time. Thus, there is a need for a more reliable pacemaker technique for detecting the onset of a ventricular arrhythmia whenever it might occur, and for providing appropriate pacing therapy.
It is an object of this invention to provide a pacemaker system having the capability of detecting conditions indicative of Long QT Syndrome, and for providing appropriate pacing therapy in response to these indications. In particular, the pacing therapy is designed to respond so as to reduce disparity of ventricular refractoriness and prevent torsades de pointes, and consequent sudden death.
It is a further object of this invention to detect the onset of a dangerous ventricular arrhythmia, and to provide intervention to prevent such an arrhythmia, either during patient awakening or otherwise.
In accordance with the above object, there is provided a pacemaker system and method of pacing, having an improved arrangement for analyzing patient QT information on an ongoing basis, and for determining the occurrence of statistically significant changes in a plurality of QT parameters, thereby providing an accurate determination of when TdP or other VT is indicated. The different QT parameters are preferably analyzed cyclically, and statistical data representative of each of said parameters is recalculated following each sensed QT signal. In a preferred embodiment, QT data is compiled in a histogram form, in accordance with different rate bins, or intervals. The current QT interval is compared with the compiled mean value of QT interval for the appropriate rate bin, and it is determined whether the QT interval has increased by more than twice the standard deviation of the mean. In a preferred embodiment, similar calculations are made for measures of QT dispersion and the time derivative of QT changes in T-wave amplitude and morphology can be measured and processed in a like manner. Additionally, the pacemaker determines whether a VES has occurred, and if so, what has been the recent rate of occurrence of VESs. This data is used to calculate whether pacing at an intervention rate above the patient""s natural rate is indicated, and if so how to adjust the intervention rate. By this means, the pacemaker system provides overdrive pacing which is accurately responsive to cardiac conditions representative of ventricular tachycardia, like TdP, or another dangerous ventricular arrhythmia.